What is the average lifespan of a dynasty?

Ibn Khaldun, answered from the texts and cited to the page.

Ibn Khaldun answers

It should be known that dynasties have a natural life span, as individuals do, and that this span does not, as a rule, extend beyond three generations.1 A generation is identical with the average duration of a single individual's life — forty years, the time required for growth to be completed and maturity reached.2 Three generations, then, amount to one hundred and twenty years; and it is in the opinion of physicians and astrologers that one hundred and twenty years constitutes the natural life span of an individual as well, what they call the great lunar year.3

The first generation retains the desert qualities: toughness, savagery, the willingness to share glory and bear privation. Group feeling remains strong among them, and people submit to them.4 The second generation, having inherited political authority, begins to lose the sharpness of that conditioning. The third grows up entirely within the comforts of sedentary culture, and by then the dynasty's militia has grown soft — cowardly, lazy, caught up in the effeminacy that hadara produces, having shed the courage and manliness that made conquest possible.5

The first element of disintegration enters through the soldiers; the second enters through money, as extravagance in expenditure outpaces revenue, and the dynasty's authority contracts stage by stage into an ever-narrower circle.6 People commonly say that the life span of a dynasty is one hundred years, and this saying points to the same truth: three generations of forty years each.7

If senility overtakes the dynasty and no claimant appears to press his advantage, the end may be deferred somewhat — but if one does appear, he will find no one capable of repelling him, and when the time is up it cannot be postponed for a single hour.8 God determines night and day.

Sources

  1. as a rule no dynasty lasts beyond the life (span) of three generations.
    The Muqaddimah, Vol 1, pp. 467–468
  2. A generation is identical with the average duration of a single individual, namely, forty years, (the time) required for growth to be completed and maturity reached.
    The Muqaddimah, Vol 1, pp. 467–468
  3. in the opinion of physicians and astrologers, the natural life (span) of individuals is one hundred and twenty years, that is, the period astrologers call the great lunar year.
    The Muqaddimah, Vol 1, pp. 467–468
  4. The first generation retains the desert qualities, desert toughness, and desert savagery. (Its members are used to) privation and to sharing their glory (with each other); they are brave and rapacious. Therefore, the strength of group feeling continues to be preserved among them.
    The Muqaddimah, Vol 1, pp. 467–468
  5. New generations grow up accustomed to this situation. (At this time,) the character of the militia softens, and they lose their toughness. This makes them cowards and lazy fellows. They are caught up in the effeminacy of sedentary culture. It causes them to shed the characteristics of courage and manliness.
    The Muqaddimah, Vol 2, pp. 149–150
  6. Weakness and destruction are the results of these two elements of disintegration... the authority of the dynasty becomes narrower than it had been at the beginning.
    The Muqaddimah, Vol 2, pp. 149–150
  7. people commonly say that the life (span) of a dynasty is one hundred years. The saying means the same as what (I have just explained).
    The Muqaddimah, Vol 1, pp. 470–471
  8. When senility becomes preponderant (in a dynasty), there may be no claimant (for its power, and then nothing will happen), but if there should be one, he will encounter no one capable of repelling him. If the time is up, (the end of the dynasty) cannot be postponed for a single hour.
    The Muqaddimah, Vol 1, pp. 470–471